Diffractive Dijet Production Research Articles

Evidence for Factorization Breaking in Diffractive Low-Q2Dijet Production

We calculate diffractive dijet production in deep-inelastic scattering at next-to-leading order of perturbative QCD, including contributions from direct and resolved photons, and compare our predictions to preliminary data from the H1 Collaboration at DESY HERA. In contrast to recent experimental claims, evidence for factorization breaking is found only for resolved, and not direct, photon contributions. No evidence is found for large normalization uncertainties in diffractive parton densities. The results confirm theoretical expectations for the (non)cancellation of soft singularities in diffractive scattering as well as previous results for (almost) real photoproduction.

Results on diffraction at CDF

In run I CDF made an extensive range of measurements studying diffractive processes. In run II these measurements can be extended using improved triggering, new detectors and larger data samples. In these proceedings run II measurements of single diffractive dijet production and double pomeron exchange production of dijets are presented.

Diffractive final states with the H1 detector at HERA

Recent measurements of diffractive dijet and charm quark production in electron-proton collisions using the H1 detector at HERA are presented, where the exchanged photon is either almost real or highly virtual. The data are compared with leading and next-to-leading order QCD calculations based on the diffractive parton distributions obtained from a recent DGLAP QCD analysis of H1 inclusive diffractive deep-inelastic scattering data, thus testing QCD factorization in diffractive ep interactions.

Diffractive production of dijets by double Pomeron exchange processes

A phenomenological description of diffractive dijet hadroproduction via double Pomeron exchange is presented. This description is based on a modified version of the Ingelman-Schlein model which includes the evolution of the Pomeron structure function and corrections regarding rapidity gap suppression effects. The same quark-dominant Pomeron structure function employed in a previous report to describe diffractive dijet and W production via single Pomeron processes is shown here to yield results consistent with the available data for double Pomeron processes as well.

Factorization breaking in diffractive dijet production

We study diffractive hard dijet production, with one or two rapidity gaps, at high energies. We emphasize that both hard and Regge factorization are broken in these processes. We show that a multi-Pomeron-exchange model for screening effects gives a specific pattern for the breakdown of factorization, which is in good agreement with diffractive dijet data collected at the Tevatron.

Diffractive dijet production

We explore the diffractive interaction of a proton with an antiproton which results in centrally produced dijets. This process has been recently studied at the Tevatron. We make predictions within an Ingelman–Schlein approach and compare them to the recent data presented by the CDF Collaboration. Earlier calculations resulted in theoretical cross-sections which are much larger than those observed by CDF. We find that, after consideration of hadronisation effects and the parton shower, and using parton density functions extracted from diffractive deep inelastic scattering at HERA, it is possible to explain the CDF data. We need to assume a gap survival probability of around 10% and this is in good agreement with the value predicted by theory. We also find that the non-diffractive contribution to the process is probably significant in the kinematical region probed by the Tevatron.

Diffractive dijet production and nuclear shadowing in pA interactions

A coordinate-space description of shadowing in deep-inelastic lepton–nucleus scattering is presented. The picture in the laboratory frame is that of quark–gluon fluctuations of the high-energy virtual photon, propagating coherently over large light-cone distances in the nuclear medium. We discuss the detailed dependence of the coherence effects on the invariant mass of the fluctuation. We comment on the issue of possible saturation in the shadowing effects at very small Bjorken-x.

Observation of diffractive J/psi production at the Fermilab Tevatron.

We report the first observation of diffractive J/psi(--> mu+mu-) production in pp collisions at root square[s] = 1.8 TeV. Diffractive events are identified by their rapidity gap signature. In a sample of events with two muons of transverse momentum p(mu)T>2 GeV/c within the pseudorapidity region /eta/ < 1.0, the ratio of diffractive to total J/psi production rates is found to be R(J/psi) = [1.45 +/- 0.25]%. The ratio R(J/psi)(x) is presented as a function of x-Bjorken. By combining it with our previously measured corresponding ratio R(jj)(x) for diffractive dijet production, we extract a value of 0.59 +/- 0.15 for the gluon fraction of the diffractive structure function of the proton.

DIJET PRODUCTION BY DOUBLE POMERON EXCHANGE AT THE TEVATRON

We report the observation of dijet events with a Double Pomeron Exchange topology produced in [Formula: see text] collisions at [Formula: see text]. The events are characterized by a leading antiproton, two jets in the central pseudorapidity region, and a rapidity gap on the outgoing proton side. Results on kinematics, production rates, and comparisons with corresponding results from single diffractive and inclusive dijet production are presented.

Probabilities of rapidity gaps in high energy interactions

We show that high energy hadronic reactions which contain a rapidity gap and a hard subprocess have a specific dependence on the kinematic variables, which results in a characteristic behaviour of the survival probability of the gap. We incorporate this mechanism in a two-channel eikonal model to make an essentially parameter-free estimate of diffractive dijet production at the Tevatron, given the diffractive structure functions measured at HERA. The estimates are in surprising agreement with the measurements of the CDF collaboration. We briefly discuss the application of the model to other hard processes with rapidity gaps.

The diffractive structure function at the Tevatron: CDF results

Results on hard diffraction from CDF are reviewed with emphasis on the determination of the diffractive structure function of the (anti)proton from single diffractive and double Pomeron exchange dijet production. Comparison of the diffractive dijet results with predictions based on diffractive deep inelastic scattering at HERA shows a breakdown of conventional QCD factorization. A similar breakdown is observed within the CDF results in comparing the structure functions obtained from single diffraction and double Pomeron exchange.

The compatibility of diffractive hard scattering in [formula omitted] and ep collisions

We show that the data for diffractive dijet production at the Tevatron p p ̄ collider are consistent with diffractive deep inelastic ep data collected at HERA. The breakdown of factorization between the two data sets is naturally explained by a strong tendency to populate the rapidity gap in the p p ̄ diffractive process by secondaries from soft underlying interactions and by bremsstrahlung associated with the presence of the hard dijet subprocess.

Coherent production of hard dijets on nuclei in QCD

We formulate the perturbative QCD approach to coherent diffractive dijet production in pion-nucleon and pion-nucleus collisions at high energy. For hard dijets the Pomeron splitting mechanism in which both helicity amplitudes are proportional to the unintegrated gluon structure function of the proton and the pion distribution amplitude is shown to dominate. In nuclear diffraction multiple Pomeron splitting components are found to give antishadowing contributions at large jet momentum. To leading twist there is an exact cancelation of effects of nuclear attenuation and antishadowing/broadening of multiple Pomeron splitting contributions. The next-to-leading higher twist correction driven by nuclear rescatterings is calculable in hard QCD and proves to be numerically very large. We argue that large higher twist effects do not preclude the determination of gross features of the pion distribution amplitude. Our results on the atomic mass number and momentum dependence of dijet cross sections agree well with the preliminary findings from the E791 experiment.

Dijet Production by Double Pomeron Exchange at the Fermilab Tevatron

We report the first observation of dijet events with a double Pomeron exchange topology produced in &pmacr;p collisions at sqrt[s] = 1800 GeV. The events are characterized by a leading antiproton, two jets in the central pseudorapidity region, and a large rapidity gap on the outgoing proton side. We present results on jet kinematics and production rates, compare them with corresponding results from single diffractive and inclusive dijet production, and test factorization.

Diffractive dijet production at HERA

Cross sections for events of the type ep → eXY, where the systems X and Y are separated by a large rapidity gap and X contains two jets, are presented for both deep inelastic scattering and photoproduction, measured using data from the H1 experiment at HERA. The corrected cross sections are described by a resolved pomeron model with parton densities for the pomeron which were obtained from a QCD analysis of F 2 D(3) from H1.

Diffractive dijet production at HERA

Interactions of the type ep → eXY are studied, where the component X of the hadronic final state contains two jets and is well separated in rapidity from a leading baryonic system Y. Analyses are performed of both resolved and direct photoproduction and of deep-inelastic scattering with photon virtualities in the range 7.5 < Q 2 < 80 GeV2. Cross sections are presented where Y has mass M Y < 1.6 GeV, the squared four-momentum transferred at the proton vertex satisfies |t| < 1 GeV2 and the two jets each have transverse momentum p T jet > 5 GeV relative to the photon direction in the rest frame of X. Models based on a factorisable diffractive exchange with a gluon dominated structure, evolved to a scale set by the transverse momentum $$\hat p_T $$ of the outgoing partons from the hard interaction, give good descriptions of the data. Exclusive $$q\bar q$$ production, as calculated in perturbative QCD using the squared proton gluon density, represents at most a small fraction of the measured cross section. The compatibility of the data with a breaking of diffractive factorisation due to spectator interactions in resolved photoproduction is investigated.

Diffractive dijet production at HERA

Interactions of the type ep → eXY are studied, where the component X of the hadronic final state contains two jets and is well separated in rapidity from a leading baryonic system Y. Analyses are performed of both resolved and direct photoproduction and of deep-inelastic scattering with photon virtualities in the range 7.5 < Q 2 < 80 GeV2. Cross sections are presented where Y has mass M Y < 1.6 GeV, the squared four-momentum transferred at the proton vertex satisfies |t| < 1 GeV2 and the two jets each have transverse momentum p T jet > 5 GeV relative to the photon direction in the rest frame of X. Models based on a factorisable diffractive exchange with a gluon dominated structure, evolved to a scale set by the transverse momentum $$\hat p_T $$ of the outgoing partons from the hard interaction, give good descriptions of the data. Exclusive $$q\bar q$$ production, as calculated in perturbative QCD using the squared proton gluon density, represents at most a small fraction of the measured cross section. The compatibility of the data with a breaking of diffractive factorisation due to spectator interactions in resolved photoproduction is investigated.

Measurement of Diffractive Dijet Production at the Fermilab Tevatron

We report the observation and measurement of the rate of diffractive dijet production at the Fermilab Tevatron p¯p collider at s=1.8TeV. In events with two jets of ET>20GeV, 1.8 0, we find that the diffractive to nondiffractive production ratio is RJJ=[0.75±0.05(stat)±0.09(syst)]%. By comparing this result, in combination with our measured rate for diffractive W boson production reported previously, with predictions based on a hard partonic pomeron structure, we determine the pomeron gluon fraction to be fg=0.7±0.2.

Azimuthal angles in diffractive

We investigate azimuthal correlations in deep inelastic diffractive scattering, e + p -> e + \tilde{p} + X. The dependence of the ep cross section on the angle between the lepton plane and some direction in the hadronic final state can be written in a simple form; its measurement can be used to constrain the cross section for longitudinally polarised photons. Using the model of nonperturbative two-gluon exchange of Landshoff and Nachtmann we calculate the distribution of the azimuthal jet angle in diffractive dijet production and find that useful bounds on the longitudinal cross section for such events might be obtained from its measurement. We then discuss the predictions of this model for the dependence of the ep cross section on the azimuthal angle of the proton remnant \tilde{p}, which contains information about the helicity content of the pomeron.

Diffractive production of dijets at HERA

We investigate the production of a quark-antiquark pair in diffractive photon-proton scattering, approximating soft pomeron exchange by the exchange of two nonperturbative gluons. In deep inelastic scattering at HERA, events with two jets and the scattered proton in the final state are predicted to be observable, with an important contribution from charm production. For photoproduction of light quark jets with high transverse momentum we find that both exchanged gluons must have a large invariant mass, so that the cross section is very small, whereas for charm quarks it is quite appreciable. From our calculation we also extract the quark structure function of the pomeron for the scaling variablez no too close to 0 or 1, finding a strong flavour dependence and a behaviour somewhat harder thanz(1−z) for light quarks.